Recently, a close proximity wireless communication technique has been developed. The close proximity wireless communication technique enables communication between two devices placed close to each other. Each of the devices having the close proximity wireless communication function includes a coupler. When the two devices are brought into proximity within a communication range, the couplers of the two devices are electromagnetically coupled with each other. Therefore, these devices can wirelessly transmit/receive signals to/from each other.
For example, a typical coupler includes a coupling element, an electrode pole, a resonant stub, a ground plane, and the like. A signal is supplied to the coupling element via the resonant stub and the electrode pole. As a result, an electric current flows in the coupling element, which generates an electromagnetic field around the coupler. This electromagnetic field enables an electromagnetic coupling between the couplers of the two devices placed in proximity to each other.
The characteristics of the coupler are affected by a distance between the coupling element and the ground plane, e.g., the length of the electrode pole. When the distance between the coupling element and the ground plane is too short, a portion of the electromagnetic field is likely to flow into the ground plane via the space due to the coupling between the coupling element and the ground plane. Accordingly, energy loss occurs to reduce the electromagnetic coupling between the couplers.
When the coupling element and the ground plane are positioned such that the distance between the coupling element and the ground plane is long, the coupling between the coupling element and the ground plane can be avoided. However, when the distance between the coupling element and the ground plane is long, the size of the coupler, i.e., the height of the coupler, increases.
Jpn. Pat. Appln. KOKAI Publication No. 2006-197449 discloses an antenna including a radiating conductor, two short-circuit pins, and a ground plane conductor. In this antenna, the radiating conductor is designed to have an axisymmetric shape with respect to a perpendicular line passing through a center axis of the ground plane conductor, so that the antenna achieves a low profile.
In Jpn. Pat. Appln. KOKAI Publication No. 2006-197449, however, energy loss caused by the coupling between the coupling element and the ground plane conductor is not taken into consideration. Therefore, it is necessary to realize a new technique in order to make a coupler having a low profile while reducing energy loss.